In recent years, water-absorbent resin particles are being applied in an even wider range of fields including hygienic materials such as disposable diapers and sanitary articles, agricultural materials such as water-retaining materials and soil conditioners, and industrial materials such as water blocking materials for cables and dew-catchers, as well as animal excreta treatment materials such as pet sheets and toilet formulations for dogs or cats, portable toilets, aromatic agents, absorbent drip sheets for meats, formulations for moisturizing cosmetics, and the like. Water-absorbent resin particles to be used for such application must have, for example, high water absorption capacity and an excellent water-absorption rate, as well as a median particle size suited for the application.
In the field of water blocking materials for cables, it is essential to rapidly arrest penetration of seawater. Also, forceful excretion of relatively large amounts of body fluids is an issue in fields of special hygienic materials such as adult diapers, incontinence pads, toilet-training pants and extra-high-absorbent napkins, pet sheets, portable toilets and the like. For such applications, particular attention has been directed toward improving absorption capacity and absorption rate. Absorption capacity can be adjusted by changing the amount of water-absorbent resin particles used in the absorbent article, but the absorption rate usually depends on the characteristic properties of the water-absorbent resin particles. Therefore, much research has therefore been carried out in the past with the aim of achieving an excellent water-absorption rate for water-absorbent resin particles.
However, reducing the particle size of water-absorbent resin particles to increase the water-absorption rate tends to impair their flow property and interfere with their handling as a powder. In addition, even if the water-absorption rate is excellent, if a relatively large number of coarse particles are present, i.e. if the particle size distribution is wide, the thickness tends to be non-uniform for water-absorbing articles whose thickness has been reduced. Because diffusion of water-absorbent resin particles is difficult and product thicknesses become non-uniform most notably with sheet-like articles that have water-absorbent resin particles sandwiched between nonwoven fabrics, such as with water blocking materials for cables, the particle size distribution of the water-absorbent resin particles should be as narrow as possible.
Examples of aqueous polymerization methods have been proposed as methods for increasing the water-absorption rate of water-absorbent resin particles while ensuring, a suitable particle size and a narrow particle size distribution, such methods including one in which a porous resin obtained in the presence of a foaming agent are crosslinked near the surface (see Patent Document 1), and one in which nitrogen air bubbles are introduced into a monomer comprising a fluorine-based surfactant for the polymerization (see Patent Document 2). Other proposals involve reversed-phase suspension polymerization, such as a method wherein a acrylic acid/acrylic acid salt water solution is subjected to reversed-phase suspension polymerization in the presence of a surfactant having an HLB value of 8 to 12 (see Patent Document 3), and a method of polymerizing a water-soluble ethylenically unsaturated monomer in the presence of water-absorbent resin particles with a different water-absorption rate (see Patent Document 4).